I'd like to see some data with the infection and death numbers broken down by age. The one reason I would begin to worry earlier than some of you is age. While the number is arbitrary, I have seen "60" as the cutoff where death goes way up - moreso than the regular flu, and gee, I just turned 60. They are using "under 40" for "likely don't need to worry much".
Without bragging, I think I'm in much better shape than the average 60 year old, and maybe the average 40 year old, but there might be factors other than health that come into play with age.